Thermal Layout Considerations for Integrated FET Chargers (Part 2)

This is part two of the layout session, thermal layout. We'll be talking about PCB thermal characteristics, and first talk about conduction concepts. Modeling temperature rise-- OK, whether it was in the past with the large TO-3 cans or TO-220 in an insulator, thermal conductive insulator in a heat sink, or if it's today's miniature components with IC soldered down to a copper plane, the thermal modeling is the same.
So in the model below, you see that you've got a current source going and resistors. The current sources represented the power. So if you dissipate 2 watts of power, that would be 2 amps of current. In the resistance, RJC is a thermal resistance in degrees C per watt. Each one of these resistances represents the path from the heat source to the ambient.
So we've got the path from the junction of the IC to the power pad of the IC junction in this case. We've got through the solder a resistance, through the PCB, and then off the PCB into the ambient. And ambience is the ground symbol. So you can see this current going through the resistance has different voltage drops, which represent temperature rise.
So the basic path of the heat is from the source of the component. You spread it out, first going through the vias and into the ground planes. Spread the heat out, and then it goes to the surface, and then off the surface into the ambient.
Thermal connectivity of other materials-- so we've got several different materials here. And you can see the thermal resistance is equal to the length of the conduction divided by sigma, which is the thermal conductivity, divided by the area. So since sigma is in the denominator, we want as large a value to get the smallest thermal resistance. So you see silver's very good. That's expensive. And copper is not too much worse. So it's pretty good.
That's why we use copper. And you can see the tin-lead, the solder, is about 10 times more thermal resistive than the copper. Epoxy, you look there, it's over 1,000 times worse than copper. But the epoxy board is a lot thicker than the copper. So it turns out in reality it's about 50 times worse on the board. And so you get 98% of the cooling from the copper, the spreading of the heat from the copper, and only about 2% from the FR4.
Thermal resistance of vias-- here we get a via. We have a length l, which is a 60 mil board. It's an 18 mil via. You plug in the numbers, and it comes out to be 100 degrees C per watt. So if your device is putting out 1 watt, and you have one via, and all the heat is going through that, that's 100 degree C rise. So that's not good enough. So you put multiple vias in. And one of our examples shows 16 vias. So you can divide that down.
And notice you can easily cut it in half by just reducing the thickness of the board by half. So remember to keep your board a minimum thicknesses. This helps you also electrically on your loop areas.
OK, lateral heat flow through material-- so now we've got the heat that started in the IC. It's coming down through the vias and connects to the layers. Now over on the right hand side, in the English units, we have copper, which you plug in the numbers.
And this is a square of copper. So we make the length and width the same, and they drop out of the formula. And it calculates out to be 40 degrees C per watt per square. And if you look down below, a 60 mil FR4 board is 2,400 degrees C per watt. And that's about 50 times worse.
So basically it's not going to do any good to transfer the heat through the FR4. The only way to spread the heat out is with copper. And you have to spread the heat out to get it off the board. The temperature rise we'll see later is a function of the area that the heat is dispersed over.
OK, we've got a copper plane here. Thermal resistance of a gap-- so we've got a copper plane here you tied your hot component to. And the heat is starting to spread out. But somebody drops down and routes through and cuts up the copper plane. Well, just a 10 mil gap increases the thermal resistance 23 degrees C per watt. So you don't want to cut up your copper plane. You want to look at your copper plane you're depending on cooling for and make sure there's no cutouts that impede the flow of the heat.
Thermal resistance for FR4-- you can see here what most people don't realize is that for a square inch, you go in it for 1 watt. It's only 8 degrees C rise. That's for a 60 mil board. If it's a thinner board, like 30 mils, it would be half of that. So if you spread the heat out, if you use the copper to spread the heat out, it will go through the board quite well.
Now, the good thing about this is if you have a PCB with components on both sides so you don't have full copper on top or bottom, you can put the copper planes inside the board, and you don't pay but a few degrees penalty temperature rise.
Talk about convection concepts-- here is convection. You notice a formula up here. Delta T-- the temperature rise is proportional to the power in the numerator. Of course if you increase the power, the temperature goes up. That makes sense. Divided by the area-- the more area you have for cooling, the larger the area, the smaller the temperature rise, divided by the heat transfer coefficient. Notice there is no value in here for materials. So it really doesn't matter. When you're talking about convection, whatever the temperature is on the surface is proportional to how much heat you're going to get rid of.
What matters is how you spread out the heat. And the copper spreads out the heat. And once it gets spread out, getting the heat off the surface is independent of the material. So you look down here below, and you see the factor is that for 1 watt dissipated over 1 inch squared, it's 166 degrees C rise. This sounds quite hot. But we'll see later it's quite reasonable. And it could be a little bit lower than that if you have any airflow or radiation. But this 166 is ignoring those factors for now.
Typical thermal requirements-- here's an example. Ambient temperature is 70 degrees C, maximum semiconductor temperature, 125 C. You're going to dissipate up to 2 watts in the semiconductor. The theta JC is 2.3 degrees C per watt.
So you do the calculations, max 125 minus what drops in the IC, which is about 5 degrees C. So the bottom of this IC or the top of the board has to be around 120 degrees C. And then the ambient can be a maximum of 70. So you're allowed across the board a 50 degree C rise.
So now we take our formula and solve for surface area. Plug in 2 watts on the top and 50 degrees C rise on the bottom, come out with 7 square inches. Well, that TO-220 can nor the IC has 7 square inches. So we've got to hook up a heat sink that has equivalent to 7 inches to meet our requirements of dissipating 2 watts with only a 50 degree C rise.
Here's an example using the formula. And it seems to be consistent with what we know. You've got a 2 and 1/2 inch board by 2 and 1/2 inch board. That's 6 and 1/4 square inches times two sides, 12.5 square inches. And you dissipate 2 watts over it. And you can see there's about a 33 degree C rise in temperature.
Air can make a big difference. But a lot of times our boards are packaged in in cases, plastic cases and stuff, so there's no air movement, or very little. So you can see here in the right figure that with no airflow at all you've got a 25 degree C temperature.
If you just go 88 linear feet per minute, or 1 mile per hour, that drops down to about 20 degrees C. That's a 20% drop in temperature. And if you go 4 or 5 miles an hour, it will drop down to 10 degrees C, which is a 60% drop in temperature. So airflow really makes a huge difference.
Strategy for cooling-- the temperature rise is a function of the power dissipated divided by the area. So you want first to have an efficient circuit. Do what you can. If it's a power converter, do what you can to optimize efficiency. And then you want to put it on a board with a good copper plane so you can conduct the heat out to a large area. And then you'll have a small temperature rise.
So to do this, you need to use 2 ounce coppers. One layer is mandatory. Two layers is better. Connect them together with vias to spread the heat out. As I mentioned before, internal planes are very effective. And you pay very little penalty once you get the heat spread out to get the heat to the surface and off the board through convection. And use both sides of the boards to cool. Avoid breaks in those planes, because it reduces the area.
OK, some examples of PCB boards-- now here's a board that's 3 inches by 0.75 inches. You notice on the top we can't do anything about this. It's a quad package. So the power pad really doesn't connect to any copper plane on the top layer. So you're really not directly going to get any help from the top layer.
And then the bottom layer, you can see there's 16 vias in the pad in the shaded area. But unfortunately they have some complex planes and routing and vias and everything else that cut up all the area. So we only have 1/4 inch by 1/2 inch, which turns out to be 1/8 inch square area, which is no area at all.
You look at the internal layers of this design, and the first inner layer has a little more area where it can spread out. But basically 50% of the board is cut off. And you don't have that much area. And then the bottom internal area has this same issue. It's routing all around. It's a very small area. So this overall board only has a 1/2 inch to 3/4 inch of cooling capability.
Now we look at the same IC put on the EVM we put out. This is a bq24295. And we have the same issue on the top layer. It's a quad package, and really very little cooling on the top. But if you look at the bottom layer, you've got a bunch of vias going in.
And this is only a two layer board. So one layer, the bottom layer, is a ground plane. And I did have to route in the ground plane a little bit. But I tried to keep them all away from the heat flow so the heat can flow outward from the power pad in all directions. It's impeding a little bit in the direction of the southeast. But it still can flow around those components and still utilize that portion of the board.
Look at some thermal images of the same board-- the top photo is the top of the board. The bottom photo is the bottom of the board. The hottest point is right where the IC is. And it's around 84 degrees C on top and bottom.
You see the bottom of the board. Overall temperature is about 10 degrees, or 9 degrees hotter than the top of the board. So the heat goes to the bottom before it spreads out and then makes its way to the top of the board.
Here's another example with 0.7 watts dissipation. Let me go back to the previous slide, and you can see that this is 2.23 watts of dissipation. And I had-- if you go back one more slide and look, this was supposed to be a high density layout example for a 1 by 2 board that was the typical requirement for this application.
So I purposely cut the plane with a slice 2/3 of the way down. And that dimension, area of that upper plane, is 1 by 2, or 2 square inches. So I purposely sized the copper plane for a typical application. And you can see that I'm dissipating 2.23 watts in this area, and the hottest board temperature is 84 degrees C. There's probably another 10 degree C rise between the bottom of the board vias and the junction temperature of the IC.
OK, we'll talk about the layout now. You can see that on the bottom is the 3D example. And you can see over to the northeast portion, or the VBUS input, and then the PMID, which is capped right by the switcher. And the distance is only 1/8 of an inch from those caps to the IC where the switching FETs are.
And then over on the right hand side, northeast side, is the inductor. And then right below it are the output caps. So everything, the whole power stage layout, is about 3/4 inch by 1/2 inch. And the high frequency area is probably 1/4 inch square. And you can see up there in the top layer, top figure, I drew out the input caps connected to the switching FETs and the IC. It goes out through the inductor and back through the caps.
Electrical and thermal layout summary-- OK, placement of your components is crucial in getting low impedance and low noise layout. So in order to do this, you've got to understand your circuit-- where the pulse currents go, where they travel. So wherever they start from, like the input cap, have them go through where they need to go and return as quickly as possible, keeping this loop area very small. You want to keep the di/dt signals of pulse currents in small loops and keep them from any other loops. The dV/dt electric coupling, you want to keep the area small and away from high impedance circuits.
Ground planes, full ground planes, give an overall lower impedance, which, when you have noise currents in them, give overall lower noise. Identify hot components and make sure there's a good copper plane to connect to, and that you can transfer the heat in all directions at least an inch or inch and a half. You have to get the heat spread out to avoid temperature rise. And you can only do that by good copper planes. Multiple planes is better, and connect them with multiple vias.
A good layout makes for a successful design. So this layout is important as any other design component. In fact, it might be more important. If you choose the wrong switching FET or wrong output cap, you can easily pop it off the board and put a new one on. But if you go through a design and build 100 boards and decide your PCB is no good, it's pretty hard to replace all the components on every board. You end up just scrapping the board. So you need to do a good job on the layout.
The power supply-- somebody knowledgeable needs to do the placement and oversee the routing. This is not black magic, but understanding the AC and DC parasitics, grounding, and cooling makes a successful design. Thank you.

Description

December 3, 2013

One of the most often overlooked design component in electronic systems is the PCB, which is as critical as any other component. Designers should use the same care with PCBs as with IC and FET switch designs. In this two-part lecture, we cover PCB electrical characteristics (AC and DC parasitics, grounds) and thermal characteristics (conduction and convection concepts).